With respect to the synthesis of alkylhalosilanes, Rochow first disclosed in U.S. Pat. No. 2,380,995 direct synthesis reaction between metallic silicon and alkyl halide in the presence of a copper catalyst. Since then, there have been reported a number of research works relating to various co-catalysts used together with copper catalysts, reactors, additives used during reaction, and the like. In the industrial synthesis of organohalosilanes, the selectivity of diorganodihalosilane which is most widely used in silicone resins, the formation rate of silanes, and the percent conversion of metallic silicon into useful silane are crucial. The selectivity of diorganodihalosilane is evaluated in terms of a weight or molar ratio of dialkyldihalosilane to the silane product and a T/D ratio.
Organohalosilane products contain diorganodihalosilane (D), triorganohalosilane (M), organotrihalosilane (T), etc. as well as by-products such as organohydrodihalosilane (H) and organohalodisilane. In particular, disilanes are known as a high-boiling fraction among silicone manufacturers using direct method organohalosilanes because few processes are available for the effective utilization of disilanes, and most disilanes are discarded as residues. The T/D ratio is a compositional ratio of organotrihalosilane to diorganodihalosilane in the entire organohalosilanes produced, with a lower T/D ratio being preferred. The formation rate of organohalosilane is represented by a space time yield (STY) which is the weight of crude organohalosilane produced per unit time relative to the weight of metallic silicon held in the reactor. In order to improve the content of diorganohalosilane produced, reduce the T/D ratio or increase the STY, various research works have been made with a focus on the catalyst and co-catalyst.
USSR Application Specification No. 617,569 (Certificate of inventorship No. 122,749) dated 24 Jan. 1959 discloses reaction in the presence of metallic silicon-copper alloy with 20 to 40 ppm of antimony added. Allegedly, the dimethyldichlorosilane content is improved from 40% to 60%. U.S. Pat. No. 4,500,724 discloses use of a copper/zinc/tin catalyst containing 200 to 3,000 ppm of tin, thereby achieving an improvement of T/D to 0.037. Japanese Patent Publication (JP-B) No. 6-92421 discloses reaction using copper arsenide having an arsenic concentration of at least 50 ppm. It is described in these patent references that reactivity, more specifically the rate of reaction of metallic silicon is improved by adding these tin, antimony and arsenic co-catalysts to a reaction contact mass comprising metallic silicon and copper.
USSR Application Specification No. 903,369 (Certificate of inventorship No. 178,817) dated 2 Jun. 1964 discloses that a co-catalyst selected from the group consisting of zinc, bismuth, phosphorus (200 ppm), arsenic, tin, and iron improves the dimethyldichlorosilane content to 72.1% from the value achieved by the above-referred Application Specification No. 617,569 (Certificate of inventorship No. 122,749). Also USSR Application Specification No. 1,152,943 (Certificate of inventorship No. 237,892) dated 20 Nov. 1969 discloses to add a phosphorus-copper-silicon alloy to a contact mass so as to give 2,500 to 30,000 ppm of phosphorus, thereby improving the dimethyldichlorosilane content to 82.3%. Moreover, U.S. Pat. No. 4,602,101 corresponding to JP-B 5-51596 discloses that 25 to 2,500 ppm of a phosphorus compound capable of generating elemental phosphorus in the reactor is added to a contact mass. Although the results of reaction according to this US patent are improved over the last-mentioned USSR patent, there still remain many problems including hazard imposed by spontaneously igniting elemental phosphorus and increased cost of raw materials. Then this US patent is also unsuitable to apply to commercial scale reactors. Also, F. Komitsky et al., Silicon for the Chemical Industry IV, Geiranger, Norway (1998), page 217, proposes the addition of phosphorus in the form of copper phosphide, leaving problems including a low percent conversion, ineffective utilization of phosphorus, and difficult control of a phosphorus concentration. U.S. Pat. No. 6,025,513 intends to add boron to a contact mass wherein the boron concentration is controlled so as to improve productivity. U.S. Pat. No. 5,059,706 discloses to introduce a phosphorus compound in a vapor phase into a reactor for increasing selectivity. U.S. Pat. No. 6,005,130 discloses to introduce organomonophosphine for increasing selectivity.
However, the phosphorus base additives used in the prior art have an outstanding trade-off between activity and composition selectivity. In particular, it is pointed out that oxide originating from phosphorus can exacerbate flow on the particle surface. Therefore, the conventional phosphorus base additives offer few merits on the continuous operation of commercial scale reactors. Other additives are known from L. Rosch, W. Kalchauer et al., Silicon for the Chemical Industry IV, Sandefjord, Norway (1996) wherein monomethyldichlorosilane is introduced for improving activity. This additive is effective only at the initial period, but not regarded as exerting a lasting effect during the continuous operation of commercial scale reactors.
While most prior art proposals focus on the elements of which the catalyst is made, as found in the foregoing references, some proposals to improve catalysis have recently been made from a brand new point of view. For example, U.S. Pat. No. 6,686,312 (corresponding to JP-A 2000-254506) discloses the use of a thermally active metallic copper powder having a large quantity of strain energy; and U.S. Pat. No. 6,365,766 (corresponding to JP-A 2000-296334) discloses the use of a copper powder in the form of flakes or scales, both for industrially advantageous preparation of organohalosilanes.
Aiming to establish a catalyst system in which both the chemical action and powder properties of a powdered catalyst contribute to improved productivity, U.S. Pat. No. 6,506,923 (corresponding to JP-A 2002-241384) proposes preparation of organohalosilanes using a catalytic metal or alloy powder as atomized.
The inventors proposed in U.S. Pat. No. 6,288,258 (corresponding to JP-A 2001-122880) efficient preparation of organohalosilanes at a reduced T/D ratio of trioganohalosilane to diorganodihalosilane by adding phosphor bronze to the catalyst.
JP-A 2000-176296 discloses a method of preparing a contact mass for organohalosilane synthesis comprising the step of applying high shear forces to metallic silicon particles and metallic copper particles in a non-oxidizing atmosphere for rubbing the particles together, thereby forming a metallic copper thin film on at least part of surfaces of metallic silicon particles. U.S. Pat. No. 6,395,917 (corresponding to JP-A 2002-128786) discloses a process of preparing organohalosilanes in which shear forces are applied to a mixture of catalyst and/or co-catalyst particles and finely divided silica for mutually rubbing the particles, thereby producing the catalyst and/or co-catalyst having finely divided silica attached to surfaces thereof, which is used in the contact mass. Additionally, JP-A 2003-313191 proposes the preparation of organohalosilanes by using a fine dispersion of a cesium-containing compound in a contact mass. None of these methods are fully satisfactory in formation rate, useful silane yield, catalyst longevity, silicon reaction yield or the like.